ILOG CPLEX 11.0 User's Manual

iteration. When you adjust barrier parameters, always check their impact on this number. Atdefault output settings, this number is reported at the beginning of each barrier optimizationin the log file, as explained in Cholesky Factor in the Log File on page 204.Another important performance issue is the presence of dense columns. A dense columnmeans that a given variable appears in a relatively large number of rows. You can checkcolumn density as suggested in Nonzeros in Lower Triangle of AA T in the Log File onpage 203. Detecting and Eliminating Dense Columns on page 210 also says more aboutcolumn density.In adjusting parameters, you may need to experiment to find beneficial settings because theprecise effect of parametric changes will depend on the nature of your LP problem as well asyour platform (hardware, operating system, compiler, etc.). Once you have foundsatisfactory parametric settings, keep them in a parameter specification file for re-use, asexplained in Saving a Parameter Specification File on page 18 in the reference manualILOG CPLEX Interactive Optimizer Commands.Memory Emphasis: Letting the Optimizer Use Disk for StorageAt default settings, the ILOG CPLEX barrier optimizer will do all of its work in centralmemory (also variously referred to as RAM, core, or physical memory). For models toolarge to solve in the central memory on your computer, or in cases where you simply do notwant to use this much memory, it is possible to instruct the barrier optimizer to use disk forpart of the working storage it needs, specifically the Cholesky factorization. Since access todisk is slower than access to central memory, there may be some lost performance by thischoice on models that could be solved entirely in central memory, but the out-of-core featurein the barrier optimizer is designed to make this trade-off as efficient as possible. It generallywill be far more effective than relying on the virtual memory (that is, the swap space) ofyour operating system.To activate the out-of-core feature, set the memory emphasis parameter to 1 (one) instead ofits default value of 0 (zero).◆◆MemoryEmphasis (bool) in Concert TechnologyCPX_PARAM_MEMORYEMPHASIS (int) in the Callable Library◆ emphasis memory in the Interactive OptimizerThis memory emphasis feature will also invoke other memory conservation tactics, such ascompression of the data within presolve.Memory emphasis uses some working memory in RAM to store the portion of the factor onwhich it is currently performing computation. You can improve performance by allocatingmore working memory by means of the working memory parameter.◆◆WorkMem in Concert TechnologyCPX_PARAM_WORKMEM in the Callable Library208 ILOG CPLEX 11.0 — USER’ S MANUAL